The freight tram is actually 100 years old--they were used
in Porto around 1900 to transport fish and coal through
Porto's steep streets. The old trams can still be seen
at the excellent but small tram museum in Porto.

There is, of course, no reason you couldn't use trams
to provide metro-freight service. The only problem that
I see is that these vehicles are probably not capable
of transporting full-sized standard sea containers,
which somewhat restricts the flexibility of the system.